This invention relates to copper alloys having satisfactory strength, electrical conductivity and stress relaxation characteristics that are suitable for use as materials for connectors and other electrical or electronic components, as well as small Young's modulus.
With the recent advances in electronics, the wire harnessing in various machines has increased in the degree of complexity and integration and this in turn has led to the growth of wrought copper materials for use in connectors and other electrical or electronic components.
The demands required of materials for connectors and other electrical or electronic components include lightweightness, high reliability and low cost. To meet these requirements, copper alloy materials for connectors are becoming smaller in thickness and in order to press them into complex shapes, they must have high strength and elasticity, as well as good electrical conductivity and press formability.
Specifically, electrical terminals must have sufficient strength that they will not buckle or deform during connection and disconnection or upon bending, as well as sufficient strength to withstand caulking of electrical wires and connector fitting followed by holding in position. To meet this need, electrical materials for use as terminals are required to have a 0.2% yield strength of at least 600 N/mm2, preferably at least 650 N/mm2, more preferably at least 700 N/mm2, and a tensile strength of at least 650 N/mm2, preferably at least 700 N/mm2, more preferably at least 750 N/mm2. In addition, in order to prevent chain transfer of deterioration that may occur during pressing, terminals must have sufficient strength in a direction perpendicular to that of working operations such as rolling. To meet this need, electrical materials for use as terminals are required to have a 0.2% yield strength of at least 650 N/mm2, preferably at least 700 N/mm2, more preferably at least 750 N/mm2 and a tensile strength of at least 700 N/mm2, preferably at least 750 N/mm2, more preferably at least 800 N/mm2, in the perpendicular direction.
Further, in order to suppress the generation of Joule's heat due to current impression, electrical materials for use as terminals preferably have a conductivity of at least 20% IACS. Another requirement is that the materials have great enough Young's modulus to ensure that connectors of small size can produce great stress in response to small displacement but this has increased rather than reduced the production cost of terminals because the need for closer dimensional tolerances has required rigorous control not only in mold technology and pressing operations but also over variations in the thickness of strip materials to be worked upon as well as the residual stress that develops in them. Under these circumstances, it has become necessary to design a structure that uses a strip material of small Young's modulus and which undergoes a large enough displacement to allow for substantial dimensional variations. To meet this need, electrical materials for use as terminals are required to have a Young's modulus of 120 kN/mm2 or less, preferably 115 kN/mm2 or less, in the direction where they were wrought and a Young's modulus of 130 kN/mm2 or less, preferably 125 kN/mm2 or less, more preferably 120 kN/mm2 or less in the perpendicular direction.
The above situation has become complicated by the fact that the frequency of mold maintenance accounts for a substantial portion of the production cost. One of the major causes of mold maintenance is worn mold tools. Since mold tools such as punches, dies and strippers wear as a result of repeated punching, bending or other press working operations, burring and dimensional inaccuracy will occur in the workpiece. The effect of the material itself on the wear of mold tools is by no means negligible and there is a growing need to reduce the likelihood of the material for causing mold wear.
Connectors are required to have high resistance to corrosion and resistance to stress corrosion cracking. Since female terminals are subject to thermal loading, they must also have good anti-stress relaxation characteristics. Specifically, their stress corrosion cracking life must be at least three times as long as the value for the conventional class 1 (specified by Japanese Industrial Standard, or JIS) brass and their percent stress relaxation at 150° C. must be no more than one half the value for the class 1 brass, typically 25% or less, preferably 20% or less and more preferably 15% or less.
Brasses and phosphor bronzes have heretofore been used as connector materials. The lower-cost brass, even if its temper grade is H08 (spring), has a yield strength (proof stress) and a tensile strength of about 570 N/mm2 and 640 N/mm2, respectively, thus failing to satisfy the above-mentioned minimum requirements for yield strength (≧600 N/mm2) and tensile strength (≧650 N/mm2). Brass is also poor not only in resistance to corrosion, resistance to stress corrosion cracking, but also in anti-stress relaxation characteristics. Phosphor bronze has good balance between strength, resistance to corrosion, resistance to stress corrosion cracking, and anti-stress relaxation characteristics; on the other hand, the electrical conductivity of phosphor bronze is small (12% IACS for spring phosphor bronze) and an economic disadvantage also results.
Many copper alloys have been developed and proposed to date with a view to solving the aforementioned problems. Most of them have various elements added in small amounts such that they keep in a balance between important characteristics such as strength, electrical conductivity and stress relaxation. However, their Young's modulus was as high as 120-135 kN/mm2 in the direction where the alloy was wrought and in the range of 125-145 kN/mm2 in the perpendicular direction. In addition, their cost was high.
Under these circumstances, researchers are most recently having a new look at brass and phosphor bronze because they both have small enough Young's moduli (110-120 kN/mm2 in the direction where the alloy is wrought and 115-130 kN/mm2 in the perpendicular direction) to meet the aforementioned design criteria. Thus, it is desired to develop a copper alloy that is available at a comparable price to brasses and which exhibits a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction in which the alloy is wrought while exhibiting a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in the perpendicular direction.
Connector materials are given Sn plating in an increasing number of occasions and the usefulness of alloys is enhanced by incorporating Sn. Inclusion of Zn as in brasses increases the ease with which to produce alloys having a good balance between strength, workability and cost. From this viewpoint, Cu—Zn—Sn alloys may well be worth attention and known examples are copper alloys having designations ranging from C40000 to C49900 that are specified by the CDA (Copper Development Association), U.S.A. For example, C42500 is a Cu-9.5Zn-2.0Sn-0.2P alloy and well known as a connector material. C43400 is a Cu-14Zn-0.7Sn alloy and used in switches, relays and terminals, though in small amounts. However, little use as connector materials is made of Cu—Zn—Sn alloys having higher Zn contents. In other words, increased Zn and Sn contents lower hot workability and unless thermo-mechanical treatments are properly controlled, various characteristics such as the mechanical ones desired for the connector materials cannot be developed and, what is more, nothing has been known about the appropriate Zn and Sn contents and the conditions for producing the desired connector materials.
Specific examples of copper alloys containing more Zn than C42500 include C43500 (Cu-18Zn-0.9Sn), C44500 (Cu-28Zn-1Sn-0.05P) and C46700 (Cu-39Zn-0.8Sn-0.05P) and they are fabricated into sheets, rods, tubes and other shapes that only find use in musical instruments, ships and miscellaneous goods but not as wrought materials for connectors, particularly as strips. Even these materials fail to satisfy all requirements for connector materials, representative examples of which are as follows:    (1) that they have a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where the alloy was wrought;    (2) that they have a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to the one where the alloy was wrought;            (3) that they have good press formability; and        (4) that they have high resistance to stress corrosion cracking.        